The present invention generally pertains to holding a sensing device against a person or animal. More particularly, the present invention pertains to sensing devices for which the performance depends at least partly on the contact pressure of the device to the body. The present invention is particularly, but not exclusively, useful for long duration measurement of biopotentials on the skin.
There are many current and emerging technologies that require contact between a sensing device and a living person or animal. In many cases, the performance of the device depends on the pressure that is used to hold the sensing device against the subject. Examples include sensors that measure biopotentials, sensors that record temperature or sound, and thermoelectric generating devices that produce electrical power via heat conducted from the skin. In all cases a tradeoff is made between the discomfort that arises due to the pressure holding the sensor against the body and the quality of the electrical, thermo, acoustic, or other coupling.
In the specific case of biopotential sensors, the standard practice is to utilize a conducting electrolyte, typically a gel, between the part of the sensor that collects the signal (often termed the electrode) and the skin. The gel provides a low impedance electrical contact which allows relatively simple amplification electronics to be used. In addition, the fluid nature of the gel allows the electrode to move slightly away from the subject without breaking electrical contact, thereby reducing the pressure that is required to ensure a reliable coupling of the signal. Similar gels can be employed to improve thermal and acoustic contact. Application of gels is currently the standard method used in clinical and research biopotential measurement applications due to the relatively low cost of the electrode and gel, their relatively long history of use, and the fact that the technique requires only a low level of training to ensure the electrodes have reliable coupling. There are, however, certain disadvantages with this technique. Specifically, the gels begin to cause skin irritation and become uncomfortable after about one day of use, limiting the capability for long-term biopotential recording. Further, a large amount of gel is needed when hair is present, or the subject must be shaved, which is both time and manpower intensive and unpleasant for the subjects. In addition, gels or other conducting fluids dry out and must be replaced. Because of these limitations, biopotential measurements are typically performed by trained staff in clinical settings, on partly unclothed subjects, for short periods of time.
An alternative type of biopotential sensor utilizes a surface electrode that does not require an electrolyte fluid or gel. These electrodes are referred to as dry electrodes and typically employ an impedance transformation using active electronics to accommodate the high impedance electrical contact that is made to the skin when fluids are not used. Typically, a dry electrode is a conductive material which is placed in direct contact with the skin and relies on a combination of resistive and capacitive coupling to the local skin potential to receive its signal. More recently, dry electrodes that rely entirely on capacitive coupling to the local skin potential have been developed. Dry electrodes offer considerable benefits in ease of use, comfort and the capability for long-term operation over many days to monitor chronic disease and health status of workers in hazardous environments.
Heretofore, active dry and insulated electrodes have not typically exhibited the same consistency and signal-to-noise ratio (SNR) as wet electrodes. In particular, dry electrodes are strongly affected by small displacements away from the skin. For dry electrodes with a conducting surface, the signal is mostly lost if the electrode is moved away from the skin by only a few microns. For capacitive electrodes, the coupling is proportional to the inverse of the separation distance between the sensor and the skin. In numerical terms, the coupling is typically reduced by a factor of about 10 as the electrode moves from a position of contact with the skin to a stand-off distance of only about 100 μm. This sensitivity to small displacements has largely prevented the use of dry biopotential electrodes.
Biopotential and also thermal and acoustic coupling is typically affected by bone and tendons in the immediate vicinity. As a result, such signals are typically gathered from areas of skin adjacent to soft tissue. Given the inherent pliable nature of soft tissue and variations in local body curvature, it is difficult to make reliable physical contact. Further, when subjects move, inertial forces can act to pull the sensor away from the skin. Thus, it is difficult to ensure reliable coupling of a sensor for detection of the variable of interest (potential, temperature, sound etc.) without applying a large force to ensure the sensor is held against the subject. However, application of pressure to the skin, particularly on the head, can quickly lead to intolerable discomfort. Thus, there immediately arises a trade-off between comfort due to the contact pressure of the sensor on the skin and the quality of the physical contact. This trade-off is particularly difficult to make for dry biopotential electrodes owing to their greater sensitivity to displacement from the skin.
The standard method to mount biopotential electrodes and most other sensors is to stick them to the skin using an adhesive, thereby avoiding the need to apply pressure. In standard clinical settings, the adhesive is typically no more uncomfortable than the electrolyte, and patients with a large amount of hair are often shaved, which helps the adhesive to attach. Further, if the adhesive contact breaks, technicians are generally on hand to reattach the electrode, and the recordings are less than a day in duration so the requirements for adhesive durability are not severe. However, one of the principal goals of using dry electrodes is to provide the capability for comfortable, long-term biopotential recordings. Another is to enable a system that can simply be put on as an item of clothing. Using an adhesive to hold the sensors in place runs counter to both of these goals. Therefore, to realize the benefit of a dry electrode, a system and method is needed to hold the electrode against the subject in an adequate, reliable pressure controlled way that does not lead to discomfort. A system and method that could provide this capability would have application to other sensors and other instances in which a mechanical interface to the skin is desired.
An alternate way to hold sensors in place against the body is to employ a strap or set of straps that encircle the torso, head or limbs. These straps contain elasticized sections so that they are in tension when in use. Generally, the straps come in a range of sizes or include an adjustable section so that the tension can be set within a desired range regardless of the size of the subject. Simple mechanics means that the tension force, t, in the straps is predominantly parallel to the surface of the body while the component of the force normal to the body is a small fraction of t. To produce an appreciable force normal to the body in order to hold a sensor against the skin, it is necessary to locally deform the straps so that, in the vicinity of each of the sensors, the straps bend away from the body, thereby producing a force in the normal direction. The magnitude of this force depends strongly on the local curvature of the body, making it difficult to set accurately in advance. Secondly, as the strap system is put on, it is difficult to ensure that the relatively large lateral forces present in the straps exactly cancel at each sensor. The resulting unbalanced force is transferred to the sensor, which in turn produces a shear force on the body that is amplified in its discomfort impact on the subject by the relatively small size of the sensor. Further, in the case of biopotential measurements, shear forces stretch the skin, which can create electrical recording artifacts via the piezo electric properties of the skin. In addition, the combination of the unbalanced tension force at the outside of the sensor and the shear force where it touches the body causes a tipping force that can compromise the desired sensor coupling to the body. These inherent problems with strap-based methods have limited their widespread adoption.
A further method to hold sensors in place against the body is to mount the electrode at the end of a sliding mechanism and use a compressed spring to provide a force to push the sensor against the subject. A defect of this approach is that the fixed end of the spring itself must be attached relative to the subject by some means. Such means need a level of flexibility in order to be comfortable and to accommodate the range of subject sizes and subject movement when in use. Thus, the spring that pushes the sensor towards the subject is itself anchored to the subject by a structure that is in some way elastic (e.g., the straps described above). Tension forces in the mounting structure act to oppose the compression force in the spring, reaching an equilibrium when they are equal. The result is that the force applied to hold the sensor against the subject is equal to the component of the tension normal to the body, which is difficult to control, as described above.
One way to minimize discomfort due to lack of control in producing a normal force when holding the sensor against the subject is to make the sensing surface itself mechanically compliant. In the case of biopotential sensors, electrically conducting rubber and foam infused with a conducting fluid has been used. However, utilizing a compliant material typically requires a compromise in the quality of the desired physical coupling and, generally, does not provide sufficient control of the applied pressure. For example, simply contacting the skin by a rubber pad still allows shear forces to be transferred to the skin in the immediate vicinity of the measurement.
In light of the above, it is an object of the present invention to provide a mounting method in which a controlled force is established to hold a sensor against the skin. The magnitude of the force of the sensor against the skin can be set to provide the optimum trade-off between discomfort experienced by the subject and the pressure necessary to provide adequate coupling for the sensor to operate as required. It is a further object of the present invention that the force should be fixed at the time of manufacture, and need not be adjusted for each subject to account for variations in body size and shape. It is yet another object of the present invention that the applied force does not change significantly due to the typical variations in body curvature between subject and variations in curvature that arise during normal bodily movement (e.g., breathing and walking).